Piezoelectric crystal filter with inductive shunt



N. M. RUST June 18, 1940.

PIEZOELECTRIC CRYSTAL FILTER WITH INDUCTIVE SHUNT Filed April 21, 1937 2 Sheets-Sheet 1' msoumcr F INVENTOR NOEL M. RUST BY 7%? ATTORNEY June 18, 1940. N T 2,204,702

PIEZOELECTRIC CRYSTAL FILTER WITH INDUCTIVE SHUNT Filed April 21, 1957 2 Sheets-Sheet 2 I |45qkd I I INVENTOR -300 000 100 +50 +100 +200 +300 NOEL M. RUST crass ABOVE AND 05100 450 KC av yfg g W ATTORNEY Patented June 18, 1940 UNITED STATES PIEZOELECTRIC CRYSTAL FI LTER WITH INDUCTIVE SHUNT Noel Meyer Rust, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application April 21, 1937, Serial No. 138,109 In Great Britain April 29, 1936 I 3 Claims.

This invention relates to coupling circuit arrangements of the kind including. a coupling element constituted by a piezoelectric crystal, and has for its main object to provide improved circuit arrangements of the kind wherein the carry-through effect of the crystal may be accurately balanced out or alternatively made to produce a desired definite predetermined effect on the operating characteristics of the circuit arrangements as awhole. The invention is of wide application and may be applied to crystal filters generally, e. g., to crystal filters for use in high selectivity intermediate; frequency circuits for superheterodyne receivers and to high selectivity circuits for automatic tuning (automatic frequency control) of radio receivers. As will be seen later, crystal coupling circuit arrangements in accordance with the invention maybe accurately designed to give almost any desired form of in balance or out-of-balance response curve.

i {The invention provides improved piezoelectric crystal coupling circuit arrangements of thesecalled bridge type but whereas most known bridge type crystal circuits are essentially capacity bridges (the coupling element which wholly or partly counterbalancesthe crystal unit in the bridge being a condenser) according to this invention a mutual inductance-i. e., magnetic coupling-is employed to counterbalance the crystal unit. It should be pointed out that the term counterbalance as employed in the last sentence is not intended to imply that there shall be exact balance; as will be seen later, the counterbalancing mutual inductance may be made an exact balance, or more than an exact balance, or less than an exact balance, according to the nature of the response curve required, 1. e.,. in dependence upon. design requirements.

Indeed, one of the major advantagesof the inin connection with the accompanying diagrammatic and graphical drawings in which:

Figure 1 shows a circuit diagram which representsthe electric equivalent of a piezoelectric crystal, i a Fig 2 shows a plot. of susceptance and conductance as obtained from the circuit of Fig. 1,

,Fig. 3 shows a foundation circuit on Which my invention is built,

Fig. 4 shows schematicallythe elementary portions'of the circuit in Fig. 3, l i i Fig. 5 shows a response curve appropriate to Fig. 4, g l y Fig. 6 is a circuit diagram representative of a preferred embodiment of my invention,

Fig. 7 shows schematically the elements of Fig. 6, Fig. 8 shows an: alternative equivalent to that ofFi-g.7, a l

Fig. 9 shows a second alternative equivalent to that of Fig. '7, l a

Fig. 10 shows a device for-matching the impedance of a piezoelectric crystal, a

Figs. 11 and 12 show respectively star and delta circuit equivalents of Fig. 10,

Fig. 13 shows a circuit embodying resistance cancellation, and i Fig. 14 shows a response curve appropriate to Fig. 13. i

For the purposes of the present invention a piezoelectric crystal may be regarded as elec trically equivalent (within a limited range of frequencies) to a circuit as shown in Fig. 1 and comprising a large inductance L in. serieswith a small condenser C1 and with a resistance R, the series sequence of these three elements being shunted by another condenser C2. To take. a practical example a typical 4.50 k. 0. crystal unit was found to be approximately equivalent to a circuit comprising an inductance L of about 53-54 henries, in series witha condenser C1 of about .0023 micro-micro-farad, in series with a re-. sistance R of about 20,000 ohms (this represents the mechanical damping of i the crystal), the shunt condenser C2 being about .32 micro-microfarad plus the cross-capacity due to the crystal holder. If, for such a circuit, conductance and. susceptance 'curves be plotted as shown in Fig. 2 with frequency along the abscissa line, the total conductance curve A will, of course, have its peak at the resonant frequency F0 of the crystal and will approach the abscissa line asymptotically on either side of this frequency; the susceptance curve 13 due to the seriesinductance Land capacity C1 alone will pass through zero at a resonant frequency F0 and will after reaching a maximum also approach the abscissa line asymptotically on either side of the resonant frequency; the susceptance curve 0 due to the carry through (i. e., the susceptance of the equvalent shunt condenser C2) will be a line substantially parallel to the abscissa line. within the working range of afrethereby; Generally speakingfat any frequency". at which the admittance is large, there will be quencies under consideration; and the total susceptance curve D will, therefore, be displaced relatively to the abscissa line, the said curve pass-- ing through zero at two frequencies X, Y, other than the resonant frequency. This has for its effect to render the total admittance curve of the crystal asymmetrical.

, Now, if a crystal be used as an interconnecting link between any two parts of an electrical cir cuit, i. e., as a so-called crystal gate," its efiectiveness in transferring ele ctricalenerg-y from one part to the other at any particular frequency 1 depends on the relationship of the admittance of the equivalent electrical circuit, representing the action of the crystal, to the electrical constants of the two parts of the circuit interconnected a relatively large transfer of energy and vice versa despite any modifying effect imposed by the impedance-frequency characteristics of the circuit elements interconnected by the. crystal. The present invention, by providing a susceptance counterbalancing that due to the carrythrough eiiect, and which causes asymmetryof the admittance curve, enables the said curve to be made substantially completely symmetrical or the asymmetry may be controlled within a wide range to any desired extent.

' Thus, consider a circuit as shown in Fig. 3 and comprising two parallel tuned circuits 'ICi, TCz, interconnected by means of a crystal PE, the first circuit TC]. being energized by a high frequency pentode. valve V1 and the second circuit energizing the grid of a subsequent valve V2. The equivalent circuit may be represented as shown in Fig. 4 by an A. C. generator G giving an output of e and connected in series with an impedance p across a network consisting of two equal shunt branches each of impedance Z1 equal to the impedance of a parallel tuned circuit TCi or TCz (assumed to be identical) and a series branch of impedance Zc which is theequivalent r crystal impedance, e being the voltage applied to the grid of the first valve V1, and its magnification factor. If E be the voltage on the grid of the second valve V2, .1. e., the voltage developed across the second shunt branch Z1 then,

,E 2, Z.+2z1

where e is, of course, the voltage at the grid of the valve V1 and g is the mutual conductance of the same valve. a

The approximation assumes that p the internal impedance of the valve, is much greater than Z1.

Such assumption is generally regarded as admissible since p in the case of a tetrode or pentode Will'be of the order of one megohm, whilst, Z1 will usually be of the order of 50,000 ohms or less.

In cases where selectivity is more important than sensitivity the value of Z1 should be made much less than any value achieved by Zc over the range of frequencies in question. On this assumption where Yc is the total admittance of the crystal. The response curve E in these circumstances will be as shown in Fig. 5 asymmetrical, having a peak value substantially at the resonance frequency F0 of the crystal and a valley at Q due to a minimum in the total admittance curve near the point remote from the resonant frequency at which the total susceptance curve crosses the abscissa line. The response curve becomes asymtotic to a line C substantially parallel to the abscissa line and representing the carrythrough effect.

.The asymmetrical conditions thus produced may in certain instances be usefully employed but it is, of course, necessary that the effect obsusceptance be equal but of opposite sign to the carrythrough susceptance, the carry-through would be balanced out and the valley would disappear, the resultant curve becoming symmetrical. Alternatively, if a negative susceptance equal to-twice the value of the carry-through susceptance be employed, the valley in the response curve will appear at an equal distance on the opposite side of the resonance frequency, the resultant carry-through effect being of the same order of magnitude asbefore,

If the-net susceptance at resonance be small compared with the carry-through susceptance, the-resultant carry-through will be small and the valley far removed from the resonance point. As the net susceptanc'e is increased the valley moves nearer to theresonance frequency and the carry-through increases until with a relatively large netsusceptance there is a large carry through accompaniedby' a very sharp change from pe'ak to valley. The response curve will be symmetrical if the net susceptance at resonance is zero. e

The net susceptance at resonance may obviously be increased in'a' positive sense merely by placing a condenser in parallel with the crystal.

The present invention is primarily concerned, 5

however, with means for making this net susceptance zero or negative.

One arrangement, in'acc'ordance with the in vention, whereby this may be effected in the above described intervalve coupling arrangement of Fig. 3 wherein a crystal PE is utilized to couple the tuned output and input circuits T01, T02 of the two valves V1, V2 in question, consists in providing a variable magnetic coupling between the inductances of the two tuned circuits. This arrangement is shown in Fig. 6- where M is the nrutual inductance provided by the magnetic coup mg.

If L be the value of each of the two inductances and M the value of the mutual inductance in Fig- 6 the equivalent circuit maybe represented as shown in Fig. 7 by. a Y or star inductance network the upper limbs of which are of value L -M and the central lower limb of which is of value This Y or-star network may be replaced as shown in Fig. 8 by a A or mesh network having inductance values shown {in Fig. 9 by two tuned circuits each having an'inductance of value L+M coupled by the This holds good whether M be positive or negative andif M be small compared with L,

is large and will be negative when M is negative. Thus, if the magnetic coupling be such that the value and sign of M can be varied, it is easily and conveniently possible to vary the response characteristics of the circuit to meet widely varying requirements. Very small values of M will be required andusually M can be neglected in comparison with it Since the range offrequencies over whichthe crystal is efiective ispof course, relatively very small, the parallel susceptance provided by the mutual inductance is substantially proportional to M. In this way it is a simple matter to balance out toa desired degree the carry-through susceptance.

In order to match the crystal impedance to that of the tuned circuits, the terminals of the crystal PE may be tapped down on the inductances of the tuned circuits TCi, TCz. A circuit in which this expedient had been adopted and by means of which experimental data have been obtained, is shown in Fig. 10 and comprises a high frequency pentode V1 having in its anode circuit a parallel tuned circuit TCi comprising a main tapped inductance in series with a small coupling inductance, said two inductances having a variable condenser connected in parallel therewith. An exactly similar parallel tuned circuit TC: is connected between the grid and cathode of a valve voltmeter (not shown). The

main tapped inductances are separately screened and a variable magnetic coupling M is provided between the coupling inductances, said inductances being located in common screening means and having electrostatic shielding means there'- between to permit of magnetic coupling only.

. The crystal PE is connected between correspondjing taps on the two tapped main inductances.

The screening is represented by chain lines.

By applying varying frequencies to the gridof the pentode valve V1 various response curves were plotted by taking corresponding readings of output with different values of inductive coupling M and it was found possible to obtain not only a substantially perfectly symmetrical curve but almost any degree of asymmetry desired, depending on the value and sense of the magnetic coupling.

The invention is, of course, not limited to this form of circuit arrangement and the circuit elements coupled by the crystal may be of The invention is applicable to all forms of simple crystal filter arrangements where it is desired to obtain a sharp symmetrical response curve or an asymmetrical response curve in which rate of change in decibels per cycle is greater on one side than on the other, and also tocases where it isdesired to use both a peak and valley one for acceptance andthe other .for rejection of desired frequencies.

In the description so far given only what may be termed susceptance cancellation methods have been described, the mutual inductance being employed to compensate or to under-compensate or over-compensate (to a desired degree) effects of. stray. capacities. Resistance effects have been neglected sofar. The efiect of resistance in the coils of the tuned circuits of a network such as that shown in Fig. 6 (and whose equivalent circuit, neglecting resistance is shown in star form in Fig. 7) will be seen from Fig. 11, which is like Fig. 7 'exceptfor the added resistance R representing an inserted resistance which is mutual to both circuits and two resistances R (each given by the two coils. The equivalent delta circuit is shown by Fig. 12 inwhich Z3 is given by where Q is the Q value of the coils.

The reactance term is dependent in sign andvalue upon the sign and magnitude (respectively) of the mutual inductance M. The other reactance term. is always positive and usually negligible as compared to OJLZ The resistance term consists of two termsterm 'QTI which is dependent as to sign and magnitude on M and as to magnitude alone upon Q. The second term is always negative and, being dependent upon R, can be controlled by varying R. The second resistance term may, in general, be made to predominate over the first term'and controllable resistance cancellation may, therefore, be obtained.

Fig. 13 showsa circuit embodying controllable resistance cancellation means in the form of a variable resistance RX. As will be seen, except for the provision of RX and the fact that the anode circuit of V1 is shown inductively coupled to circuit T01, Fig. 13 is like Fig. 6. To take a practical example for a 450 k. 0. cycle, M may be variable from to -70 and RX may be variable from to 120 ohms, the main (uncoupled) coils in the tuned circuits TCi, TCz being of '700 micro-henries each. Varying the resistance RX practically affects only the valley depth in the response curve, having little or no effect on the peaks. The insertion of the resistance to efiect resistance cancellation produces a deep valley in the response micro-henries curve, This is shown in Fig. 14 in which the full line curve shows the result obtained with resistance cancellation and the broken line curve the way in which the valley practically disapv tube and a utilization device, said system com-- prising a tuned circuit connected to said output circuit and a second tuned circuit connected to said utilization device, each of said tuned circuits having a variable capacitor, an isolated inductance porton and a magnetic coupling portion, the two magnetic coupling portions constituting the primary and secondary, respectively of a transformer, a series'of taps disposed intermediate the terminals of said isolated inductance portions, a piezoelectric device having its two electrodes selectively connected in circuit between one of the taps of one isolated inductance and one of the taps of the other isolated inductance, and means for varying the reactance between the primary and secondary of said transformer.

2. An electric vwave filter having an asymmetrical frequency response characteristic and having a piezoelectric crystal for determining said characteristic, parallel tuned circuits each connected to its respective electrode of said piezoelectric crystal,one parallel tuned circuit having a minor portion of its inductance coupled to a similar minor portion of the inductance in the other parallel tuned circuit, and means for varying the degree of magnetic couplingbetween the two .saidI parallel tuned circuits, thereby 'toobtain a rejection frequency which in one case is considerably lower and in another case is considerably higher than the frequency of maximum response.

I 3.'A filter system inductively coupled to the output circuit of one electron discharge tube and the inductance portions of the second tuned circuit and the control grid of said second tube, a

piezoelectric device connected across corresponding points of said tuned circuits, and means for varying the response characteristic of said filter system as awhole in such manner that an asymmetrical curve of rejection frequency is obtained a;

which curve extends from a point considerably lower than the main resonant frequency through a condition in which the rejection frequency is brought close to the resonant frequency and extending on the other side thereof through a point where the rejection frequency is considerably higher than said resonant frequency.

NoiiL MEYER RUST. 

